The present invention relates to an image pickup tube which is preferably used with a target voltage enhanced, and its operating method.
Generally, a photocondutive-type image pickup tube or an X-ray image pickup tube (hereinafter generally referred to as an image pickup tube) is provided with a target section for converting an image of incident light or an X-ray (hereinafter generally referred to as light) into a charge pattern to be stored, and a scanning electron beam generating section for reading the stored charge pattern as a signal current. Immediately after the target section is scanned by the electron beam, the image pickup tube is operated so that the surface potential on the electron beam scanning side balances with the cathode potential. Incidentally, the structure and operation theory of the image pickup tube are disclosed in detail in e.g. SATSUZO KOGAKU (or Imaging Engineering) by Ninomiya, et al published by Corona-sha (1975), pp. 109 to 116.
If excess secondary electrons are emitted in such an image pickup tube when the scanning side of the target section is scanned by the electron beam, its surface potential immediately after scanned will not become the cathode potential. Thus, the image pickup tube cannot perform its normal operation. JP-A-48-102919 (laid-open on Dec. 24, 1973) discloses that in order to reduce the secondary electron-emission yield, an electron beam landing layer of porous Sb.sub.2 S.sub.3 is provided on the scanning side of the target section.
Further, excess electron beams once reflected from the target section may be reflected by the electrode within the tube to be incident on the target section again; thus, a spurious signal will be produced to be superposed on a video signal. As means for restraining such an undesired phenomenon, (1) JP-A-61-131349 (laid-open on Jun. 19, 1986) discloses that an additional conductive layer is provided in the non-scanned region on the photo-conductive film surface of the target section, and (2) JP-A-63-72037 (laid-open on Apr. 1, 1988) discloses that the transparent conductive layer of the target section is divided into that in the effective scanned region and that in the non-scanned region on a substrate, and these transparent conductive layers are connected with different power supplies so that they are individually controlled by the power supplies.
Further known are techniques of providing a thick photo-conductive layer in order to improve the sensitivity of an image pickup tube or reduce the capacitive lag, and of using the avalanche multiplication phenomenon in the photo-conductive layer in order to further enhance the sensitivity of the image pickup tube. These techniques are disclosed in e.g. National Convention Report of 1982 of The Institute of Television Engineers of Japan, pp. 81 to 82 by Kawamura, et al, and IEEE ELECTRON DEVICE LETTERS EDL-8 No. 9 (1987), pages 392 to 394. These image pickup tubes must be used with an enhanced voltage (hereinafter simply called a target voltage) between a target electrode and a cathode electrode. Such a use is likely to produce a phenomenon that a distortion-in-picture-image or shading is generated on a reproduced image, or an abnormal pattern varying in a waterfall shape is generated in the peripheral portion of the reproduced image (hereinafter simply called a waterfall phenomenon), and to produce another phenomenon that the signal level of the video signal corresponding to a part of the reproduced image, particularly its peripheral portion is drastically reduced or the polarity of the video signal is inverted (hereinafter simply called an inversion phenomenon). As means for restraining these undesired phenomena, (3) JP-A-1-298630 (laid-open on Dec. 1, 1989) discloses that the secondary electron emission yield in the non-scanned region on the scanning side of the target section is made lower than that within the effective scanned region, and (4) JP-A-2-204944 (laid-open on Aug. 14, 1990) discloses that an insulating thin film is provided outside the effective scanned region of the target section.
The image pickup tube fabricated using the above prior arts (3) and (4) can restrain the undesired phenomenon such as the above waterfall phenomenon and inversion phenomenon in a region up to a relatively high target voltage. However, if the image pickup tube is used with a higher target voltage in order to enhance its sensitivity, the undesired phenomenon such as the above waterfall phenomenon and inversion phenomenon will occur again.
The image pickup tube fabricated using the above prior art (1) is so designed that the conductive layer provided in the non-scanned region on the photo-conductive film side of the target section is kept in contact with the target electrode through the photo-conductive film. The resistance of the photo-conductive film will be decreased by incident light. Therefore, the enhanced target voltage causes charging between the target electrode and the additional conductive layer so that the photo-conductive layer may be injured. As a result, the target voltage cannot be enhanced sufficiently.
Further, the image pickup tube fabricated using the prior art (2) is so designed that the transparent conductive layer of the target section is divided into that in the effective scanned region and that in the non-scanned region on a substrate by the photo-conductive film. Therefore, the image pickup tube according to the prior art (2) provides the same problem as that according to the prior art (1); the target voltage cannot be enhanced sufficiently. Further, the process of fabricating the target section is complicate, and so during the fabricating process, dust is likely to be applied to the target and minute defects is likely to occur there. This will provide local image defects, thereby reducing the production yield. Accordingly, the highly sensitive image pickup tube cannot be provided so that a highly image pickup device and a highly sensitive camera cannot be realized.